1. Technical Field
The present invention relates to an optical receiving apparatus, a test apparatus, an optical receiving method, a testing method, a test module, and a semiconductor chip. More particularly, the present invention relates to an optical receiving apparatus that outputs a data value of digital data transmitted by an optical signal and to a test apparatus that tests a device under test such as a semiconductor circuit.
2. Related Art
Conventionally, an optical receiving apparatus used for optical communication converts a weak photoinduced current generated by a photodiode that receives an optical signal into a voltage signal through an impedance conversion circuit. Because of the small current value of the current, a current path from the photo diode until the impedance converting circuit is a place in which the S/N ratio of the signal decays remarkably. Furthermore, resistance to common mode noise is low.
Furthermore, because a pulse output by a laser diode on a side of an optical transmission apparatus has rising and falling times that are asymmetrical, timing jitter increases because of intersymbol interference. In addition, because temperature drift in an optical system is large in comparison to an electronic circuit, a clock data recovery type (CDR type) is often employed and data is normally transmitted with an AC connection type. Because of this, encoding is necessary so that the low frequency cutoff of the transmission system is not exceeded.
The aforementioned problems cause a limited bandwidth of the optical receiving apparatus, an increased size of the transmission electronic circuit, and overall complication. Where the size of the signal processing circuit is large for every channel, cost-performance is worsened in a case where optical transmission is executed in parallel.
As opposed to the problems in such optical transmission, in a case of electrical transmission, a circuit that calculates charge on a side of the receiving circuit to decrease intersymbol interference is known as in, for example, Japanese Patent Application Publication No. 2005-25768, pgs. 17-18, FIG. 15. Such a circuit decreases the intersymbol interference by obtaining a difference between the charge charged by a previous cycle of a signal waveform and the charge charged by a present cycle of the signal waveform.
In conventional optical transmission, however, the aforementioned problems of intersymbol interference, noise, and low frequency cutoff of the transmission system are not solved.